1. Field of the Invention
The present invention relates to a method of producing nuclear fuel pellets. Particularly, this invention relates to a method of producing U3O8 powder having large surface area and small particle size by oxidizing defective UO2 pellets, and manufacturing nuclear fuel pellets which are stable in a pore structure and high in density through the use of a mixture comprising UO2 powder and U3O8 powder. Description of the Related Art
Nuclear powder utilizes heat generated by nuclear fission. An industrially applicable nuclear fuel material includes cylindrical or circular pellets produced by molding and sintering an oxide such as uranium (U), plutonium (Pu) and thorium (Th) alone or by combination.
The nuclear fuel material currently in widest use is uranium oxide pellets. In a typical production process of the uranium oxide pellets, a lubricant is added to and mixed with a starting material of uranium oxide powder, and then pre-molded under a predetermined pressure, e.g., about 1 ton/cm2 to produce a slug. The slug is pulverized to obtain granules. Subsequently the lubricant is added to and mixed with the granules obtained and then compression-molded to form a compact, i.e., green pellets having a theoretical density (TD) of about 50%. The compact is sintered in a hydrogen-containing gas atmosphere to produce uranium oxide pellets. The uranium oxide pellets obtained as described above have a TD of about 95.5% and a grain size of 6 to 10 μm. Crystal grains of the nuclear fuel pellets are of an equiaxed polyhedron.
A general process of producing the nuclear fuel pellets typically entails a certain amount of defective loss. For example, surfaces of the nuclear fuel pellets produced by sintering are ground to suit dimensions of a drawing of a technical specification, bringing about grinding-induced byproducts. Also, during the pellet production process, any defects of the pellets may lead to defective products failing to satisfy the drawing of the technical specification. A defective loss such as defective UO2 pellets or grinding sludgy generated during sintering is oxidized in the air at a temperature of 400 to 600° C. and converted into U3O8 powder. Then the U3O8 powder is mixed with the UO2 powder to be recycled. The mixture of UO2 and U3O8 powder is pressed and sintered to produce UO2 pellets in the same way as the single UO2 powder.
The U3O8 powder obtained by oxidizing the defective pellets is much less sinterable than the UO2 powder. Therefore, the nuclear fuel pellets containing the U3O8 powder recycled from defective powder or defective pellets is lower in density, smaller in a crystal grain size and more unstable in a pore structure than pellets produced from pure UO2 powder.
In a conventional case where the pellets are prepared from powder mixture of UO2 and recycled U3O8 powder obtained by oxidizing the defective pellets at a temperature of 400 to 600° C., the density of pellets are decreased in TD by about 1 to 1.5% and in a grain size by about 1 to 2 μm per 10 wt% of the U3O8 powder added. The content of recycled U3O8 powder in the mixture is limited within 15% by weight of the mixture of UO2 and U3O8 powder. Therefore, the production process of the recycled U3O8 powder needs to be regulated to improve the sinterability of U3O8 powder, thereby enhancing characteristics of the nuclear fuel pellets.
U.S. Pat. No. 4,889,663 discloses a method of producing pellets using U3O8 powder obtained from UO2 powder. In this document, the UO2 powder is oxidized to obtain the U3O8 powder and then added to the UO2 powder to be molded and sintered. The U3O8 powder oxidized from the UO2 powder is added to the UO2 powder produced by a dry process, thereby increasing strength of a compact and thus reducing end-capping.
Yet, the pellet production method as described above is accompanied with problems due to very big ring-shaped pores formed inside the pellets. The ring-shaped pores, if present, roughen surfaces of the pellets and increases volume of open pores of the pellets, which, among the pores, are connected to the surfaces of the pellets. Increase in volume of the open pores in the pellets allows moisture from the air to be more adsorbed through the open pores while the pellets are handled or stored. Moreover, the open pores serve as a passage for releasing nuclear fission gas outside the pellets. Therefore, a greater volume of the open pores degrades quality of the nuclear fuel pellets.
In a case where the U3O8 powder obtained by oxidizing the pellets or scrap powders is used to prepare the pellets, many defects arise due to difference between the U3O8 powder and the UO2 powder in characteristics. Japanese Patent Laid-open Publication No. 2000-314790 and Korean Patent No. 0424331 disclose improved technologies, in which post-oxidized U3O8 powder is pulverized up to a certain size and recycled. Mechanical pulverizing undermines economic efficiency due to a post-oxidization follow-up process and causes particulate-induced environmental pollution. Japanese Patent Laid-open Publication No. 2000-314791 discloses a process of repeatedly oxidizing and reducing oxidized powder up to a certain size. However, this technology is burdensome to the process and low in economic efficiency.